For sensory systems, feedforward projections from thalamic relay cells provide the cortex with information about the external environment. The cortex, in turn, sends extensive feedback to thalamic relay cells. The cortex thus functions both to process information supplied by the thalamus as well as to influence dynamically the transmission of thalamic input. The primary goal of the experiments presented in this proposal is to determine the functional organization of corticogeniculate feedback projections and their influence on visual processing. The proposed study involves four sets of experiments. The first major series of experiments (Specific Aim 1) will test the hypothesis that the corticogeniculate pathway contains physiologically distinct populations of neurons that selectively innervate the magnocellular and parvocellular layers of the LGN. The remaining experiments will examine the functional role of corticogeniculate projections during visual processing. In general, proposed roles for corticogeniculate function fall into two broad categories: (1) the corticothalamic pathway serves to sharpen the receptive field properties of thalamic neurons, and (2) the corticothalamic pathway serves to enhance the transmission of sensory information from periphery to cortex. The second series of experiments (Specific Aim 2) will therefore test the hypothesis that corticogeniculate feedback sharpens LGN receptive fields by suppression from the extraclassical receptive field. The third series of experiments (Specific Aim 3) will test the hypothesis that corticogeniculate feedback adjusts non-spatial aspect of LGN responses. Finally, the fourth series of experiments (Specific Aim 4) will compare the effects of directed attention on corticogeniculate neurons and LGN neurons in order to test the hypothesis that attention and the corticogeniculate pathway influence the gain of LGN responses to visual stimuli. Given the central importance of corticothalamic pathways for governing the excitability of thalamocortical networks, it is important that we understand the functional properties of the corticothalamic pathway, as disorders of the pathway likely underlie several illnesses affecting vision and visual processing.